This application pertains to the art of cathodic protection and more particularly, to the regulation of current and voltage impressed upon a structure in order to protect the structure from corrosive effects of the environment.
The invention is particularly applicable to the protection of metal and concrete structures such as road overpasses, storage tanks, pipelines, bridges and parking garages.
Cathodic protection techniques have traditionally used a direct current impressed across the surface of a structure that is to be protected from the usual corrosion processes occurring in soil or concrete. Ideally, the current level is maintained at a value just sufficient to halt the electrochemical corrosive reactions occurring at the surface of the structure. The criterion for determining the degree of protection from corrosion is the value of the voltage of an electrochemical half-cell "barrier" existing at the structure/soil interface. This barrier is built up and maintained by passing a D.C. current through the structure/soil interface.
Cathodic protection systems have been comprised of a large 60 Hz power transformer with selectable winding taps followed by a diode rectifier bridge and a filter arrangement to produce a D.C. voltage. The selectable tap adjust system permits varying the applied D.C. voltage. This voltage is applied to an electrode arrangement buried in the ground near the structure being protected. The technique is simple and has been widely used within the industry. In these systems, the lack of electronic feedback control necessary to compensate for varying soil conditions may render the tap adjust system ineffective since the taps, if changed at all, are changed by field service personnel at infrequent intervals. Also, since such manual systems are typically inefficient and not automatically adaptable to a widely varying load, they have to be specially designed for each application, and even then, oftentimes cause unnecessary anode consumption because of their inability to assess and adapt the proper energy requirements for the system.
Another consequence of the tap adjust or phase controlled technique is the mandatory use of large 60 Hz power transformers. The transformers are used to step down the utility voltage to the comparatively low voltages used in cathodic protection. The transformer also provides safety isolation between the utility line and power supply output. These transformers are typically quite heavy and bulky. Additionally, in order to control efficiency, a different transformer must be designed and built for each different set of customer requirements. The transformer must be designed to match the utility power to the particular worst case voltage and current ranges in the customer specification. It is, therefore, difficult to use two or three standard transformer designs to meet all production demands.
Another system used in the industry comprises phase controlled rectifiers between the power transformer and the filter. Phase controlled rectifiers allow the use of electronic feedback control to regulate the current and voltage passing into the protection electrode system. At present, phase controlled rectifier circuits are the industry standard for applications requiring unattended regulation of current flow in cathodic protection systems.
Another consequence of using 60 Hz power transformers is in order to obtain reasonably low ripple (i.e. steady D.C. current) of the power supply output, some filtering must be included between the phase controlled rectifiers or diode bridge and the output to the electrodes. Since the controlled rectifiers are pulsed at line frequency (60 Hz), the filter capacitor (and possibly inductor) must be fairly large.
A third method of assessing the cathodic protection is to measure the barrier potential at the structure/soil interface. Since the barrier is really a chemical half-cell, a voltage measurement must be made using a second standard "reference" half-cell. The actual measurement of the barrier potential is made with respect to the standard reference cell.
The barrier half-cell is connected to the reference half-cell electrically by the conductivity of the soil or concrete between the surface of the steel and the active surface of the reference half-cell.
The reference half-cell is small and is usually located at a convenient location nearby the surface that is to be protected. Using a high input impedance voltmeter connected between the structure interior and the terminal of the reference cell which is not contacting the soil, the barrier potential of the structure can be measured with respect to the half-cell used. The variation in a well-built reference is very low with time and temperature.
Often, large uncontrollable errors are introduced into the measurement of the true barrier potential. The chief source of error is a voltage drop existing in the volume of the soil between the steel structure and the location of the reference half-cell. The voltage drop results from the product of electrical soil currents flowing in the vicinity of the structure - reference cell volume and the electrical resistance of the soil. This is called "IR Drop".
This IR drop represents a voltage generator separating the reference half-cell from the steel. The result is a measurement error in the voltage between the reference half-cell and the structure interior. The amount of the error is exactly equal to the IR drop in the soil between the structure and reference cell. The IR drop may be positive or negative, depending on the direction of the currents flowing in the soil.
One prime source of IR drop error is the flow of the actual cathodic protection current itself. Thus the protecting current masks the measurement of its own effectiveness. The IR drop varies with the level of the cathodic protection current and with the soil resistance. Soil resistance may change with time due to moisture variations.
One strategy for minimizing the IR drop is the placement of the reference half-cell physically close to the steel structure to be protected. This is usually inconvenient and costly especially in structures that are deep underground or have a large physical extent. The advantage is that the soil volume, and hence the soil resistance, is decreased. The voltage error would be decreased for IR drop due to cathodic protection current.
Recent research in cathodic protection chemistry indicates that a more representative measure of the "health" of the structure may be obtained by decreasing the cathodic current to zero very quickly (less than a millisecond) and taking a reading of the barrier potential of the structure within a very short interval (10's of milliseconds) after the current has been brought to zero. Due to the size of the filter components used in the phase controlled and diode bridge rectifier systems this "instant off" capability cannot be achieved, since the time required to discharge the energy contained in the filter can amount to 10's of milliseconds. In such circuits, the filter will discharge into the electrode system which thereby precludes a practical "instant off" feature. Instant off can only be achieved with rectifiers pulsing essentially directly into the electrode system with no D.C. filtering.
As noted above, an attractive alternative is the method of measuring the "true" barrier potential while the cathodic protection current is zero. Thus the protection current must be interrupted periodically and a voltage measurement taken between the reference half-cell and the structure barrier half-cell.
The resulting voltage is called the "Off Potential". The Off Potential (O.P.) is a much more representative measurement of the protection barrier voltage and an accurate representation of the health or effectiveness of the barrier. On the basis of the O.P., an electronic control system may be used to control the cathodic protection current supplied to the barrier to maintain a desired barrier potential. The desired value of the barrier potential is determined by chemical considerations and is assessed by a Chemical Engineer.
The general method of adjusting the protection current level by comparing the measured voltage of the structure-to-reference to a manually commanded or set voltage is called "auto-potential" control in the industry. As explained above, IR drops are due to the cathodic protection current itself (plus any unrelated interference currents) and may corrupt the measurement of the barrier potential.
A strong interest has developed for improved corrosion protection in concrete structures such as road overpasses, bridges and parking garages. These structures require generally more precise control of cathodic protection current and different parts of the structure must be protected by independently regulated channels.
Presently, if it is desired to protect separate zones, a structure of conventional technology requires the use of a separate rectifier for each zone of the structure. Or, dissipative rheostats must be used from a single rectifier to divide and distribute the single rectifier output to various zones of the structure. Both techniques are costly and overall inefficient. Also the rheostat approach requires periodic adjustment by service personnel.
Although switch-mode power circuit technology is conventionally known for applications such as computer-related circuit powering or other high speed electronics, it has not been employed in the cathodic protection industry. One reason for avoiding its application is that it is not expected to be useful in a broad situation that is widely variable from 0 to 100% power capacity. Also since cathodic protection is usually current intensive, those skilled in the art were normally suspect that switchmode power circuit technology which primarily useful in low current applications could be adapted for cathodic protection systems.
A conventional forward convertor switchmode power supply uses a power transformer with a power switching transistor connected to the primary and a diode rectifier connected to the secondary or output winding. The output rectifier is followed by an L-C filter to produce a smooth D.C. output during the time that the power switching transistor is ON, an input supply is connected across the primary and power is transferred to the output through the rectifier diode. When the power transistor is turned OFF, the flow of power through transformer is stopped. The output inductor current continues to flow through a catch diode. When the power transistor is turned on again, the inductor current will flow through the rectifier D1 and the secondary winding of the transformer. If we assume that the inductor current is fairly constant and greater than zero at all times, the D.C. output voltage will be equal to the (input line voltage) .times. (transformer turns ratio) .times. (duty ratio).
The duty ratio is the fraction of the time that the power transistor is on. The D.C. output voltage may be controlled by varying the duty ratio.
The forward converter is an example of a buck regulator. Other types of buck regulators include push-pull converters, half-bridge, or full-bridge pulse-width modulated circuits or even current-fed push-pull converters. The common factor in buck regulators is that the input line is chopped by a power transistor (with or without a power transformer) and the resulting chopped voltage waveform is fed to an L-C output filter. The L-C filter basically filters out the A-C part of the chopped waveforms and allows only the D.C. content of the waveform to be presented to the output. By using the power transistor duty ratio to vary the D.C. content of the chopped waveform, we can vary the D.C. output.
The above-approach works well in most power supply applications. However, at very low output load currents, the unavoidable current ripple in the output filter inductor causes the inductor current to hit zero during part of each switching cycle. This condition causes the relationship between D.C. output voltage and duty ratio to change, the net result being that the control of the output voltage becomes difficult at low output currents. In most applications a minimum load is specified to avoid this effect. Cathodic protection applications see instances where load current drawn can be significantly less than 10% of maximum output. Otherwise, unnecessary anode consumption and shorter system life is the result.
Another problem with conventional systems occurs at low load resistances approaching short circuit. If the pulse width is being controlled to produce constant output current, then as the load resistance decreases, the output voltage must decrease. To decrease the output voltage, the pulse width must be lowered. There is a limit to which the pulse width can be lowered in constant frequency pulsewidth modulated switching power supplies. It is difficult to control the output voltage below approximately 5% of maximum output voltage depending on input line and frequency of operation.
The present invention contemplates a new and improved cathodic protection system and method which overcomes all of the above referred to problems and others to provide a new protection system which improves the quality of the cathodic protection for a structure while being readily adaptable to a plurality of uses with systems for a wide variety of structural items.